Access to a communications frequency spectrum has traditionally been subject to regulation and restriction. This is for a variety of technical and legal reasons, not least the need to avoid one form of wireless communication interfering with another. In the past, when access to spectrum was not in high demand, and technical limitations prevented fine delineation of ‘zones’ of the spectrum for different communications uses, regulation was relatively tight. In recent years, this regulation has been relaxed somewhat.
Policies on the liberalisation of the communications spectrum have been formulated by various bodies, not least the United States Federal Communications Commission (FCC) in “Spectrum Policy Task Force,” ET Docket No. 02-135, November 2002.
With the liberalisation of spectrum regulations, introduction of flexible spectrum policies and the emergence of cognitive radios, many proposals have been made for cooperative sensing and collaboration for efficient spectrum utilisation by primary (licensed) and cognitive secondary (unlicensed) users of the spectrum. Examples of this can be seen in “Collaborative Spectrum Sensing for Opportunistic Access in Fading Environments,” (A. Ghasemi, E. Sousa, 1st IEEE Conference on Dynamic Spectrum Access Network (DySPAN), November 2005), “Cooperative Sensing among Cognitive Radios,” (S. Mishra, A. Sahai, R. Brodersen, International Conference on Communications (ICC), June 2006) and “Cooperative Spectrum Sensing in Cognitive Radio Networks,” (G. Ganesan, Y. Li, 1st IEEE Conference on Dynamic Spectrum Access Network (DySPAN), November 2005).
By way of background, cognitive radio is a field of wireless communications technology in which either a network on a distributed basis or a wireless node in particular can change parameters governing transmission or reception characteristics in order to establish effective communication without interfering with licensed users of a given frequency spectrum. This alteration of parameters can be based on active monitoring of several factors in the external and internal radio environment, such as reservations made of the radio frequency spectrum, user behaviour and network state.
In cognitive radio, spectrum agile radio, or 802.22 WRAN, non-contiguous portions of spectrum are identified as to be managed by a “secondary market mechanism”, as discussed in “The Spectrum Framework Review” (OFCOM, November 2004: available at www.ofcom.org.uk/consult/condocs/sfr/sfr), “Spectrum Policy Task Force Report” (Technical Report, FCC, ET Docket 02-135, November 2004) and “Facilitating opportunities for flexible, efficient, and reliable spectrum use employing cognitive radio technologies” (FCC, ET Docket 03-108, December 2003).
In the context of cognitive radio, and in particular with reference to the so called Secondary market mechanism, two sets of users can be considered, namely:                the Primary User (PU)—the licensed user of the spectrum or a user recognised as having high priority for the spectrum band, and        the Secondary User (SU)—an opportunistic user or “cognitive” user who accesses spectrum on a temporary basis when PUs are not making use of the spectrum        
The reader will appreciate that the use of this concept of division between PU and SU user devices is for the purpose of describing the present invention clearly with regard to the prior art, and an actual implementation of cognitive radio could be provided without this distinction being made, either explicitly or implicitly. Indeed, as described later, this distinction between PUs and SUs is not an essential element of the claimed invention.
In order to introduce a secondary user into a channel in a useable spectrum, the fundamental approach adopted at present is to use the available spectrum opportunistically without interfering with the primary. Secondary users (SU nodes) are allowed to use/access the spectrum when the licensed or primary user (PU) is not in use, but should exit once PU arrives. In dynamic situations, every channel is susceptible to channel degradation due to interference, or call termination due to the arrival of a primary user. So, SU nodes periodically monitor the spectrum usage and look for available free channels for transmission.
On vacating the channel required by the primary user, the secondary user scans for the next vacant channel and switches to that channel in order to resume communications. Alternatively an emergency escape route identified in advance can be used, as described in UK Patent Application GB2449224A. In that approach, secondary nodes, wanting to communicate, use the vacant channels opportunistically. These nodes can choose any channel from the multiple vacant channels that are available.
There are many proposals describing how the actual scanning process is achieved. Examples are set out in “On Detecting White Space Spectra for Spectral Scavenging in Cognitive Radios,” (F. Harris, Wireless Personal Communications, vol. 45, pp 325-342, 2008), “Candidate Spectral Estimation for Cognitive Radio,” (M. Rojas, M. Lagunas, A. Perez, Proc. of the 11th WSEAS Intnl. Conference on Communications, July 2007) and “Spectrum Scanning and Reserve Channel Methods for Link Maintenance in Cognitive Radio Systems,” (S. Subramani, S. Armour, D. Kaleshi, Z. Fan, IEEE Vehicular Technology Conference (VTC), May 2008).
The choice of scanning mechanism employed in any particular implementation is beyond the scope of this disclosure.
The scanning process can be carried out in a distributed or centralised manner. In an infrastructure based secondary network, the access point/controller generally executes the scanning process. In contrast, in ad-hoc networks, scanning is carried out in a distributed manner; when SU nodes sense the PU, they individually scan for vacant spectrum and exit communications. Carrying out the scanning process on every node individually can result in the cumulative consumption of battery power being higher than with a centralised approach. Further, the end result may not be an efficient use of the spectrum resource. In addition, individual scanning and exiting of communications might lead to disruption of ongoing communications.
There are many standard bodies focussing on opportunistic spectrum access, which have established teaching in this area. The most well established source of standard setting disclosures is that of the 802 set of networking standards by the IEEE.
802.11h: This amendment to IEEE 802.11™ specifies the extensions to the standard for wireless local area networks (WLANs) which provide mechanisms for dynamic frequency selection (DFS) and transmit power control (TPC) that may be used to satisfy regulatory requirements for operation in the 5 GHz band in Europe. The mechanisms for DFS specified in this amendment are for infrastructure based networks alone.
802.22WG: 802.22 is a new working group of the IEEE 802 LAN/MAN Standards Committee which is seeking to establish standards for the construction of Wireless Regional Area Networks (WRAN) utilizing white spaces (channels that are not already used) in the allocated TV frequency spectrum. The use of the spectrum will be in an opportunistic way in order not to interfere with any TV channel that is transmitting.
US patent application US 2008/0081675 A1 describes a communication network including a plurality of communication devices communicating over multiple systems or channels and also communicating over short range link, such as Personal Area Network (PAN) link. In order to manage access to a channel, each device uses pre-stored scan lists and cooperative scanning. The cooperative scanning involves partitioning the scan lists amongst the plurality of devices via the short range link to reduce battery consumption or enhance performance.
US patent application US 2008/0039105 A1 is concerned with determining a channel for communication, in a multi-device network. During operation of a secondary communication system, cells are formed by a plurality of localised nodes to alert other nodes within the cell of frequencies which must be protected or otherwise avoided. All nodes within the cell monitor a different subset of all available frequencies, and share information with respect to acceptable and protected frequencies with each other via low-power, short-range communication. Each node then forms a list of available channels for communication, and chooses a single node to report this information back to the controller.
US patent application US 2008/0102849 A1 describes management of operating channels of an 802.11h compliant network. When implemented on a single access point, the system autonomously adjusts the operating channel so that the network operates on the channel with the least interference. When deployed on the access nodes in a campus or urban setting, the system rapidly converges to a stable interference minimising frequency re-use pattern with the average reduction in interference realised by each 802.11 cluster in the range of 19 dB (as device density increases, the expected reduction in interference increases with the exact gain in interference reduction a function of the specific propagation environment and network topology).
Reductions in interference are also realised by legacy systems which are not implementing the algorithm, but operating in the presence of the enhanced access points. When new access points are added to the network, the network automatically converges to a near optimal frequency reuse pattern. This is accomplished without any message passing between access nodes, without any adjustments to the existing 802.11 protocol, without user guidance, without prior or externally generated knowledge of the environment or network, and with minimal additional computational complexity at the access node. The mechanism of Dynamic Frequency Selection (DFS) described in that publication, for infrastructure based networks, uses the game theoretic approach for exchange of interference information.
US patent application US 2006/0084444A1 looks at ways of using unused portions of an allocated frequency spectrum in a wireless communications system that broadcasts content to wireless stations. A first wireless station may communicate with a second wireless station on an idle broadcast channel while keeping the resulting interference level below an acceptable maximum limit at the other wireless stations. Using interference level information that are measured at the wireless stations, the wireless station can negotiate with the other wireless station on an establishment channel for subsequent communications on one or more broadcast channels. The wireless station may receive broadcast content on a time slice that corresponds to a broadcast channel and that is further processed by the wireless station. Otherwise, the wireless station can utilise the corresponding time to measure an interference level for the corresponding channel or to transmit or receive data to/from another wireless station.
As will be appreciated from the above, the conventional technology does not optimise for the secondary network's battery consumption or allow seamless communications on channel switchover.